1. Field of the Invention
The present invention relates to a process for the continuous catalytic preparation of propylene glycol carbonate (PGC) from propylene oxide (POX) and carbon dioxide (CO.sub.2), which is characterized by an adiabatic and particularly mild, energy- and material-saving procedure.
2. Description of the Related Art
Many proposals have been made for the preparation of PGC. These primarily relate to the use of certain catalysts. A process for the preparation of PGC or ethylene glycol carbonate (EGC) from POX or ethylene oxide (EOX), respectively, and CO.sub.2 is described in Chem. Ing. Techn. 43 (1971), 903. This process also underlies the publication in Fette, Setfen, Anstrichmittel 73, (1971), 396, but here, only the preparation of EGC is specifically considered. For the preparation of EGC, adiabatic temperature conditions are considered there to be technically not realizable (loc. cit. 398). In the process described there, CO.sub.2, and EOX are reacted together at 80 bar and 190.degree. to above 200.degree. C. in a reactor filled with ethylene glycol carbonate and the heat of reaction is removed with the aid of a heat transport medium circulating in counter-current, which in turn is cooled with water. Under such reaction conditions peak temperatures up to 220.degree. C. in the reactor are obtained which have a product-damaging action, which is explicitly referred to in the cited publication (p. 397), and which would be difficult to master in particular in the case of industrial plants. The entire energy of reaction is removed unused in this case. Since the reaction of POX with CO.sub.2 to give PGC has a similarly high heat of reaction as that of EOX with CO.sub.2 to give EGC and temperature peaks likewise act in a product-damaging way on the PGC, the statements from Fette, Seifen (loc. cit.) can also be applied to the preparation of PGC from POX and CO.sub.2, Adiabatic temperature conditions, in contrast to the procedure described in Fette, Seifen (loc. cit.) and Chem. Ing. Techn. (loc. cit.), are characterized in that the entire heat of reaction liberated is absorbed by the reaction mixture itself; in the case of exothermic reactions this leads to an increase in the temperature of the reaction mixture.
In the process which is described in U.S. Pat. No. 4,314,945, the reactors for the preparation of alkylene carbonates from alkylene oxides and CO.sub.2, take as a basis various combinations of flow tubes and pumped circulation reactors which are operated at 10-50 bar and 100 to a maximum of 200.degree. C. The corresponding carbonate serves in all reactor parts as reaction medium and there makes up in each case 85-99.6% by weight of the reactor contents.
The alkylene oxide is metered into the first reaction zone of such a combination jointly with the catalyst dissolved in the carbonate.
The feeding of the CO.sub.2, is carried out in the combination preferred in U.S. Pat. No. 4,314,945 of a pumped circulation reactor with downstream flow tubes at a different, remote point of the reactor. Precisely in the first reactor of such a combination, where the local alkylene oxide concentration is still particularly high in the immediate surroundings of the metering site, the type of metering selected acts disadvantageously on the selectivity: the alkylene oxide, promoted by the high local catalyst concentration and the lower CO.sub.2 concentration, can rearrange to form acetaldehyde and thus serves as a base for polycondensation reactions. It is disadvantageous, moreover, that CO.sub.2 is only fed in a slight excess with respect to the catalyst (and not with respect to the alkylene oxide). The components for the combinations, which represent a reactor, used in U.S. Pat. No. 4,314,945 are on the one hand pumped circulation reactors--comprising a stirred tank stirred by a stirrer or by the gas or liquid flowing in, having an external circulation and heat exchanger for product cooling--and on the other hand flow tubes operated under adiabatic temperature conditions and with a subsequent cooler or themselves provided with a cooler.
A pumped circulation reactor is characterized in that high pumped circulation rates are employed in the case of exothermic reactions, the heat of reaction being removed during the reaction in the reactor and in the associated heat exchanger. This procedure is completely different from the adiabatic technique in which the reaction is completed in one pass through the reactor and the entire heat of reaction is absorbed by the reaction medium.
Common to the reactor components in U.S. Pat. No. 4,314,945 is that, independently of their number and sequence within the combination, in each of them only a partial conversion of the alkylene oxide can take place since otherwise the heat problem of the highly exothermic reaction of the alkylene oxide with the CO.sub.2 is not mastered.
Only by a complex connection of these individual components, each furnished with coolers, with partial conversion can, in total, a conversion rate of alkylene oxide of at least about 99.5% and a carbonate selectivity of at least about 99% be achieved.
Use of the sensible heat of the reaction in the process itself or for producing steam is not described in U.S. Pat. No. 4,314,945.
There was the desire to develop a process which on the one hand avoids damaging temperature peaks and on the other hand uses the energy of reaction as far as possible; such a use can proceed in the context of the process according to the invention itself, for example for working up the crude product or for generating heating steam for other processes.